The ExportToImage method takes a FrameworkElement parameter. The size of the image is the same as the size of that element. If you don't want to change the size of the RadCartesianChart I can propose the following: when a chart needs to be exported, have a button that creates a new window and insert the chart into the window. You can then resize this window as you please. This window will have an export button which will export the chart. When you have finished with exporting the element, you can remove it from the window and insert it back where it was originally.
Another possible option is to change the size before exporting the chart and then change it back. Please find a sample project attached demonstrating both approaches. Do let me know if you find any of them suitable for you.
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